When I’m working with film, I have to imagine the effect of a filter before I release the shutter. A filter in front of the lens lightens its own color, darkens its opposite and leaves neutrals unchanged. Exposure has to be increased to compensate for the density of the filter.
The color circle in Figure 1 shows the scientific relationships of primary colors. Red is the opposite of cyan, yellow is opposite of blue, and magenta is the opposite of green. Knowing these relationships helps you anticipate the effect of a filter or suggest what filter will give you the effect you want.
Figure 1. This color circle shows the scientific relationships of primary colors.
With a strong red filter, for example, I can render a green leafy plant in front of a reddish sandstone wall as a dark plant against a light and delicately textured wall, as green is nearly the opposite of red. Or, with a deep green filter, I can render that same plant as very light against a dark and moody wall.
When working digitally, I make the capture in RAW and then experiment with different interpretations of these relationships using controls in Lightroom or Photoshop. Each color in the capture can be manipulated independently from other colors, and since the skewing is done in post-processing, no exposure compensation is necessary.